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How do Organisms Reproduce

Chapter: How do Organisms Reproduce

 

Introduction:

Reproduction is the process by which living organisms give rise to their off springs. Reproduction, unlike the essential life processes like nutrition, respiration or excretion is not necessary to maintain the life of an individual but still it is a very important process in the life of a living organism. Reproduction is necessary to maintain the existence of living organisms. Reproducing organisms produce new individuals which look very much like themselves.

Do Organism Create Exact Copies of Themselves ?

 Organisms look similar because of the similarity in their body design; the similarity in the body design is because the blue prints of their design are similar. Reproduction at its most basic level involves in making copies of blue prints of the body designs. Chromosomes in the nucleus of the cell contain genetic information for inheritance and it is transferred to the off-springs in the form of DNA. The DNA in the cell nucleus is the information source for making proteins. If the information is changed different proteins will be made which lead to different body designs. Therefore the basic event of reproduction is the creation of a DNA copy. DNA inside the cell replicates to produces its copies. Replication of DNA in a reproducing cell produces two copies of DNA. These DNA copies separate from each other with their own cellular apparatus. Effectively, a cell divides giving rise to two cells. The DNA copies generated after the DNA replication will be similar but are not identical to each other and this is because of the variation that occurs during the copying process of DNA. The variation that occurs during the reproduction process is the basis for evolution.

The Importance of Variation:

  1. The consistency of DNA copying during reproduction is important for the maintenance of body design features that allow the organism to use that particular niche.
  2. Niches can change because of reasons beyond the control of the organisms. Temperatures on earth can go up or down, water levels can vary, or there could be meteorite hits, to think of niches.
  3. Let us consider an example of bacteria living in temperate waters and if the water temperature increases by global warming most of the bacteria would die, but a few variants resistant to heat would survive and grow further.
  4. Variation is thus useful for the survival of species overtime.

Modes of Reproduction Used by Single Organism:

1. Fission:

  1. Most of the unicellular organisms use cell or fission as the mode of their reproduction. There are many different patterns of fission.
  2. Bacteria and protozoa split into two equal halves during cell division. In organisms like amoeba splitting take place in any plane.
  3. In organisms like Leishmania, which have a whip like structure at one end of the cell, binary fission occurs in a definite orientation in relation to these structures.
  4. In other single celled organisms like the malarial parasite, plasmodium, divides into many daughter cells simultaneously by binary fission.
  5. Yeast, on the other hand, can put out small buds that separate and grow further.

2 .Fragmentation:

Multicellular animals with simple body organization can undergo reproduction by simple methods. Fragmentation is one such simple methods of reproduction. For example Spirogyra, simply breaks up into smaller pieces upon maturation. These pieces or fragments grow into new individuals.

3. Regeneration:

Many of the fully differentiated animals have the ability to give rise to new individuals from their body parts. That is, if the individual is somehow cut or broken up into many pieces, many of these pieces grow into separate individuals. For example, simple animals like Hydra and Planaria can be cut into any number of pieces and each piece grows into a complete organism. This is known as regeneration. There are some specialized cells to carry out regeneration, they undergo proliferation and make large number of cells. From this mass of cells, different cells undergo changes to become various cell types and tissues. The sequence of events in regeneration is called as development.

4. Budding:

In this mode of reproduction a bud develops as an outgrowth at a specific site due to repeated cell division. These bud develop into tiny individuals and when they fully mature they detach from the parent and become new independent individuals.  Budding is mostly seen in hydra.

5. Vegetative propagation:

It is the mode of asexual reproduction in plants. Many parts of the plant like the root, stem, and leaves can develop into new plants under appropriate condition this is called vegetative propagation.  Plants raised by vegetative propagation can bear flowers and fruits earlier than those produced from seeds. Vegetative propagation is used in methods such as layering or grafting to grow plants like sugarcane, roses or grapes for the purpose of agriculture. One of the advantages of vegetative propagation is that all the plants produced by this method are genetically similar enough to the parent plant. Buds produced in the notches along the leaf margin of Bryophyllum fall on the soil and develop into new plant.

6. Spore formation:

In many multi-cellular organisms, specific reproductive parts can be identified. The thread like structures called hyphae have tiny bob like structures called reproductive structures. They contain cells or spores that can develop into new Rhizopus individuals. The spores are covered by thick walls that protect them until they come into contact with another moist surface and can begin to grow.

Sexual Reproduction:

The mode of reproduction in which both male and females are involved in the production of new individuals is called as sexual reproduction.

Why the Sexual Mode of Reproduction ?

In the mode of asexual reproduction because of the variations which occur during the DNA copying mechanisms there could be some errors in the population of the organisms. Every individual organism cannot be protected by variation, but in a population, variations are useful for ensuring the survival of the species. While DNA-copying mechanisms are not absolutely accurate, they are precise enough to make the generation of variation a fairly slow process. If the DNA copying mechanisms were to be less accurate, many of the resultant DNA copies would not be able to work with the cellular apparatus, and would die. Thus, two different individuals in a population would have quite different patterns of accumulated variations. Combining variations from two or more individuals would thus create new combinations of variants. Each combination would be novel, since it would involve two different individuals. The sexual mode of reproduction incorporates such a process of combining DNA from two different individuals during reproduction.

Sexual Reproduction in Flowering Plants:

  1. The reproductive parts of angiosperms are located in the flower. Stamens and carpels are the reproductive parts of the flower containing germ cells.
  2. The flowers may be unisexual or bisexual. Unisexual flowers contain either stamen or carpels but not both. Example: papaya, watermelon. Bisexual flowers contain both stamen and carpels.
  3. Stamen is the male reproductive part that produces pollen grains which are yellowish in color. Carpel is the female reproductive part and is located in the centre of the flower.
  4. The carpel is made up of three parts. The bottom swollen part is the ovary, middle elongated part is the style and the terminal part is stigma which is sticky in nature.
  5. The ovary contains ovules and each ovule has an egg cell. The male germ cell produced by the pollen grains fuses with the female gamete present in the ovule.
  6. The fusion of these gametes is called fertilization and it results in the production of zygote which is capable of growing into a new plant.
  7. The pollen has to be transferred from stamen to stigma for the process of fertilization to occur.
  8. If the transfer of pollen occurs in the same flower it is called self-pollination and if the pollen is transferred from one flower to another, it is known as cross-pollination.
  9. Cross pollination is achieved by wind, water or animals.
  10. After the pollen lands on a suitable stigma, it has to reach the female germ-cells which are in the ovary. For this, a tube grows out of the pollen grain and travels through the style to reach the ovary.
  11. After fertilisation, the zygote divides several times to form an embryo within the ovule.
  12. The ovule develops a tough coat and is gradually converted into a seed.
  13. The ovary grows rapidly and ripens to form a fruit. Meanwhile, the petals, sepals, stamens, style and stigma may shrivel and fall off.
  14. The seed containing embryo develops into a seedling under appropriate conditions and this process is known as germination.

Reproduction in Human Beings

We can observe different changes in human beings, both male and female during their teenage. These changes include development of facial hairs in men, development of breast and darkening of nipples in women, development of genital hairs in both male and females. All of these changes are the aspect of sexual maturation of the body. The period of adolescence is called puberty. Sexual mode of reproduction means that germ-cells from two individuals have to join together. This can happen by the external release of germ-cells from the bodies of individuals, as happens in flowering plants, or it can happen by two individuals joining their bodies together for internal transfer of germ-cells for fusion, as happens in many animals. For the animals to participate in mating, their state of sexual maturity must be identifiable by other individuals. Changes during puberty such as hair growth patterns are signals that sexual maturation is taking place. The transfer of germ cells in animals needs special organs for the sexual act such as erection of penis. In mammals such as humans, the baby is carried in the mother’s body for a long period, and will be breast-fed later. The female reproductive organs and breasts will need to mature to accommodate these possibilities

Male Reproductive System:

  1. The male reproductive system consists of parts which produce germ cell and also the parts which deliver the germ cell to the site of fertilization.
  2. Testes is the organ in which sperms are formed. Testes is located in the scrotum outside the abdominal cavity because sperm formation requires lower temperature than the normal body temperature.
  3. Testosterone is the hormone secreted by testes it plays an important role in sperm formation and also brings about some changes in the appearance seen in boys at the time of puberty.
  4. The sperms formed are delivered through the vas deferens which unites with a tube coming from the urinary bladder. Therefore urethra serves as a common passage for both urine and sperms.
  5. Prostrate glands and seminal vesicles present along the path of vas deferens add their secretions to sperms so that sperms are fluid in nature.
  6. The fluidic nature of sperms helps in their transport and also provides nutrition.
  7. The sperms are tiny bodies that consist of mainly genetic material and a long tail that helps them to move towards the female germ-cell.

Female Reproductive System:

  1. Ovaries are the female productive organs. Female germ cells or eggs are made in ovaries. Some important hormones are also produced by ovaries.
  2. By birth a girl contains several numbers of eggs in her ovaries but they mature when she attains puberty.
  3. One egg is produced by the ovaries every month. The egg is carried from the ovary to the womb through a thin fallopian tube.
  4. The two oviducts unite into elastic bag-like structure known as the uterus. The uterus opens into the vagina through the cervix.
  5. During sexual intercourse sperms enter through vaginal passage and they travel upward and reach the oviduct where they fuse with the egg.
  6. The fertilised egg is called zygote and gets implanted to the uterus lining and starts dividing. The lining thickens and is richly supplied with blood to nourish the growing embryo.
  7. The embryo gets nutrition from mother’s blood through placenta. This is a disc which is embedded in the uterine wall. It contains villi on the embryo’s side of the tissue. On the Mother’s side is blood spaces, which surround the villi. This provides a large surface area for glucose and oxygen to pass from the mother to the embryo.
  8. The developing embryo will also generate waste substances which can be removed by placenta.
  9. The development of baby inside the mother’s uterus will take nine months and the child is born as a result of rhythmic contraction.

What happens when the Egg is not fertilised?

If the egg is not fertilized, it lives for about one day. Since the egg is released every month the uterus also prepares itself for fertilizing the egg. The lining of uterus becomes thick and spongy which is required for nourishing the embryo if the fertilization had taken place. So the lining slowly breaks and comes out through vagina as a blood. This cycle takes place every month and is called as menstruation.

Reproductive Health:

The process of sexual maturation is gradual, and takes place while general body growth is still going on. Therefore, some degree of sexual maturation does not necessarily mean that the body or the mind is ready for sexual acts or for having and bringing up children. There can be pressure from our friends for participating in many activities, whether we really want or no. There can be pressure from families to get married and start having children. Since the sexual act is an intimate connection of bodies, it is not surprising that many diseases can be sexually transmitted. These include bacterial infections such as gonorrhea and syphilis, and viral infections such as warts and HIV. We can prevent such infections by using condom which covers the penis. The sexual act can always have chances to lead to pregnancy. We can avoid pregnancy by using contraceptives. Some of the contraceptive methods include creation of a mechanical barrier so that the sperm does not reach the egg. Another way of contraceptives is by creating a hormonal imbalance so that the eggs are not released and fertilization does not occur. Other contraceptive devices such as the loop or the copper-T are placed in the uterus to prevent pregnancy. If the vas deferens in the male is blocked, sperm transfer will be prevented. We can stop the sperm transfer by blocking the vas deferens. Surgical methods are used to block fallopian tube so that sperm does not reach the egg. We have to maintain female sex ratio for a healthy society. Although prenatal sex determination has been banned people still go for it and abort if they get to know it is a girl child.  This is decreasing the child sex ratio at an alarming rate. The size of the human population is a cause of concern for many people. This is because an expanding population makes it harder to improve everybody’s standard of living.